1. Field of the Invention
The present invention relates to a laminated decorative sheet for in-mold decorating injection molding, an in-mold decorating injection molding method using the laminated decorative sheet, and a decorative molding decorated with the laminated decorative sheet.
2. Description of the Related Art
Decorative moldings formed by decorating the surfaces of resin moldings have been used for various purposes. In-mold decorating injection molding methods are disclosed in, for example, JP-B No. Sho 50-19132 and JP-A No. Hei 11-91041. An in-mold decorating injection molding method carries out operations for forming a resin molding and bonding a decorative sheet to the surface of the resin molding simultaneously to produce a decorative molding having a decorative surface. Generally, such an in-mold decorating injection molding method uses acrylic resins, ABS resins, polystyrene resins and the like. A decorative sheet for in-mold decorating injection molding consists of a base sheet of an acrylic resin, an ABS resin or a polystyrene resin, and a pattern layer of an ink or an adhesive. The adhesive layer is formed of a styrene resin, an acrylic resin a polyvinyl chloride-acetate resin or the like.
ABS resins are used generally for forming, for example, automotive interior furnishings by injection molding. Because of the high cost of ABS resins and recent increasing concern for environmental problems, demand for using polypropylene resins including propylene copolymers instead of ABS resins has increased.
Decorative sheets for in-mold decorating injection molding proposed in Jpn. Pat. No. 2923281 and JP-A No. Hei 11-277695 have an adhesive layer formed of a chlorinated polypropylene resin that is adhesive to polypropylene resins and polyolefin resins.
Although a chlorinated polypropylene resin having a higher chlorine content and bonded to polypropylene resins by thermal bonding has a lower adhesive strength, the chlorinated polypropylene resin has bad solubility and bad compatibility and a coating liquid or an ink of the chlorinated polypropylene is unable to form an adhesive layer or the like by coating or printing unless the same is not chlorinated. A chlorinated polypropylene resin having a chlorine content of 30% by weight has minimum heat resistance. In a practical in-mold decorating injection molding process, the adhesive strength of an adhesive layer is reduced by a high process temperature. Consequently, a decorative sheet for in-mold decorating injection molding is liable to be separated from a solidified resin molding formed by injection molding by an external force that acts thereon when ejecting a decorative molding from a mold. Therefore, measures must be taken to prevent the separation of the decorative sheet from the resin molding.
When a decorative sheet of an acrylic resin capable of serving also as a protective layer for a decorative molding is used, the decorative sheet is heated and drawn in a preforming process for preforming the decorative sheet so as to conform to the molding surface of a mold, and then the decorative sheet is cooled rapidly. Consequently, a stress induced in the decorative sheet by drawing remains in the decorative sheet and contractile force is produced in the decorative sheet when the decorative sheet is heated again at a temperature near to the glass transition point of the decorative sheet. Thus, the adhesive strength of an adhesive layer of a conventional chlorinated polypropylene resin yields to the contractile force of the decorative sheet of the acrylic resin having a thickness in the range of 100 to 200 μm at temperatures around 100° C. and the decorative sheet creases or peels off. Therefore, the heat resistance of, for example, automotive interior furnishings provided with an acrylic resin sheet is not satisfactory in a working environment.